This invention is related to an optical switch array, and more particularly, is related to an optical micro-electro-mechanical switch array.
Optical switches can replace electrical switches in electro-optical systems, because of their low weight and immunity to electromagnetic interference, and because they eliminate the need for optical-to-electrical and electrical-to-optical conversion at the switch.
The importance of fiber-optic switches has been increasing due to the rapid growth of optical fiber networks. Recently, there has been a growing demand to make fiber-optic switches based on micro-electro-mechanical system (MEMS) technology. The use of MEMS techniques to make fiber-optic switches offers several advantages such as miniaturization, high performance and batch production or low cost.
One type of a conventional micromachined free-space optical matrix switch uses electrostatically actuated torsion mirrors, as shown in FIG. 1. The optical matrix switch includes a first base member 101, a plurality of bonding pads and interconnections 104, a second base member 105, and a plurality of optical fibers 108. The first base member 101 has an array of bores 103 formed therethrough and arranged in a plurality of columns and rows. The torsion mirror has a reflective panel member 102 and a torsion bar connected to the reflective panel member 102 by a connector section.
Each of bores 103 is sized to receive a respective one of the torsion mirrors. Each of the torsion mirrors is mounted onto the first base member by embedding opposite distal ends of the torsion bar into the first base member so that the torsion mirrors can pivot between a reflective state and a non-reflective state.
The second base member 105 includes an array of cavities 106. The first base member 101 and the second base member 105 are connected to each other with the cavities 106 disposed in a manner to receive an end portion of the reflective panel member 102 when the reflective panel member 102 is in the reflective state. A support wall 107 retains the reflective panel member 102 at an appropriate position for redirecting a beam of light traveling in a first direction to a second direction.
One problem with the optical matrix switch described above is that the insertion loss between the fibers is quite high due to a fact that there are no lenses for collimating the light between them.
Another problem is that precision alignment is required to connect the first base member and the second base member together so that the support wall is properly oriented to retain the reflective panel member properly in its reflective state.
Additionally, electrostatic torque causes the reflective panel member to move between the reflective state and the non-reflective state. Electrostatic torque is a complicated area of the art and there is limited data to determine when mechanical fatigue might be expected over the lifetime of the conventional optical matrix switch.
As shown in FIG. 2, another type of a conventional micromachined free-space optical matrix switch includes a base member 201, a plurality of reflective panels 202, 203, a plurality of microlenses 204, a plurality of optical fibers 205, and an actuator. Each reflective panel is pivotally connected to the base member and is unbiasedly movable between a reflective state and a non-reflective state. The actuator is connected to the base member 201 and the reflective panels and causes the reflective panels to move between the reflective state and the non-reflective state.
Each reflective panel includes at least one hinge pin member and at least two hinge pin connecting members. A staple member having a channel sized to receive at least one hinge pin member is connected to the base member 201 with at least one hinge pin member disposed within the channel so that the reflective panel can pivotally move about a pivot axis that extends through at least one hinge pin member.
The actuator includes a hinge assembly and a translation plate. The hinge assembly has at least one connecting rod with a first end pivotally connected to the reflective panel and an opposite second end pivotally connected to the translation plate. The translation plate is slidably connected to the base member and moves between a first position and a second position.
The actuator is a scratch drive actuator mechanism or a comb drive mechanism. One of these mechanisms is connected to the translation plate and is operated in conjunction with the base member to cause the translation plate to move to and between the first and second positions.
This type of optical matrix switch does not include any microstructures for receiving optical fibers and lenses. It is impossible to realize passive alignment between the optical fibers, the mirrors and the lenses.
The optical matrix switch of FIG. 2 is fabricated using a three-layer polysilicon process that can only be offered by a few MEMS technology centers. Since the three-layer polysilicon process is still not in a definition manner, even though all the mirrors go through the same processes, they still have quite different natural frequencies.
A reflection position of the optical matrix switch is established by placing at least four separated movable components in place. Any vibration may change position of the movable components and cause the switch to go out of the reflection position.
The optical matrix switch has a mirror-to-mirror distance greater than 175 micrometers. It has been shown that the mirror-to-mirror distance more than 175 micrometers results in an insertion loss between the two mirrors greater than 2.5 dB which exceeds the allowable maximum insertion loss for practical applications.
In order to solve the aforementioned problems and other problems, an optical switch array has been developed by the present invention. The optical switch array at least possesses the following features:
The reflection mirrors of the optical switch array are self-oriented to the vertical direction of its operation plane.
The reflection mirrors and their supporting flexural strips are orthogonal to each other so that bending of the strips can be turned into the vertical movement of the mirrors.
The supporting flexural strips are formed from single crystal silicon with excellent mechanical properties.
The reflection mirrors are formed from (111) silicon crystal planes with excellent optical properties.
The reflection mirrors are double-sided mirrors to increase switching density.
The optical switch array has a plurality of microchannels capable of holding optical fibers therein and realizing passive alignment between the reflection mirrors and the optical fibers.
The optical switch array has a plurality of cylindrical lenses capable of realizing passive alignment with the optical fibers and the reflection mirrors.
With the aforementioned features, the present invention provides an optical switch array comprising a plurality of (111) silicon planar plates formed in a (110) silicon substrate, bonded by two opposite (111) crystal planes vertical to the surface of the (110) silicon substrate and arranged in columns and rows. Each (111) silicon planar plate is supported by a flexural silicon strip that is anchored to the (110) silicon substrate at least at its one side. An air gap separates each silicon strip from the (110) silicon substrate and allows the silicon strip to bend up and down. Two stop shoulders are used to guide the (111) silicon planar plate vertically moving up and down and halt the (111) silicon planar plate precisely at a lifted vertical position.
The optical switch array further comprises a plurality of microchannels formed in the (110) silicon substrate. The longitudinal axis of each microchannel is oriented to a (111) silicon crystal plate at an angle of 135 or 45 degrees so that they are parallel or orthogonal to each other.
Each microchannel holds a cylindrical lens or an optical fiber therein. The optical fiber has one end bonding a cylindrical lens thereon and the other end extending to a central office that sends out optical signals.
Each (111) silicon planar plate is aimed by four cylindrical lenses. The four cylindrical lenses are arranged in X shape so that each two adjacent cylindrical lenses are orthogonal to each other and each two opposite cylindrical lenses extend along a same line.
When the optical switch array is in a non-reflective position, the (111) silicon planar plate hides or retracts into the (110) silicon substrate so that its top level aligns with the surface of the (110) silicon substrate. When a light beam comes out from a cylindrical lens that faces a (111) silicon planar plate it can travel over the (111) silicon planar plate and enter an opposite cylindrical lens without any blockade between them.
The optical switch array can be electrically driven into a reflective position. To do this, each individual silicon strip is configured to be a conductive plate of a plate capacitor. When a dc voltage is applied to the plate capacitor, the silicon strip is electrically bent up until it travels through the air gap and halts at the above stop shoulders. Consequently, the (111) silicon planar plate supported by the silicon strip is lifted out of the (110) silicon substrate so that its top portion blocks the optical travelling path between the two adjacent cylindrical lenses. When a light beam comes from a cylindrical lens, it can be reflected by the (111) silicon planar plate and go to another cylindrical lens that is oriented to the cylindrical lens at an angle of 90 degrees.
The cylindrical lenses are made of graded index fibers that have a core refractive index decreasing almost parabolically and radially outwardly toward the cladding. The diameter of the cylindrical lenses is chosen as the same as the optical fibers. The microchannels are formed so that all the optical fibers and the lenses placed in the microchannels can be aligned with each other in parallel manner or orthogonal manner.
A preferred method of the present invention for manufacturing the optical switch array includes the essential steps of selective formation and etching of oxidized porous silicon and anisotropic etching of (110) silicon substrates in KOH solution.
The (110) silicon substrate has four (111) crystal planes intersecting with its surface at an angle of 90 degree. In KOH etching, the etching rate for (111) crystal planes is much lower than the etching rate for other crystal planes. Because of these, a vertical planar plane can be formed in the (110) silicon substrate if the etch mask accurately orients to a (111) crystal plane to the proper crystal direction to minimize mask undercutting. The (111) silicon planar plate has two opposite (111) crystal planes that are atomic smooth and qualified for optical mirrors with very high quality.
Selective etching of a buried oxidized porous silicon layer is used to release the silicon strips from the (110) silicon substrate. Before etching of the oxidized porous silicon, four fabrication steps need to be carried out. The first step is to form a heavily doped layer in a lightly doped silicon substrate. The second step is to form a lightly doped epitaxial layer on the surface of the heavily doped layer. The third step is to oxidize the heavily doped layer to form an oxidized porous silicon layer. The fourth step is to remove the oxidized porous silicon layer in an etch solution used for silicon dioxide.
During the process for forming the (111) silicon planar plates, the oxidized porous silicon layer severs for an etching stop layer. Because of a small thickness of the (111) silicon planar plate, the openings of the etching mask have to align with the oxidized porous silicon layer precisely. When the etched bottom lowers down to the oxidized porous silicon layer the etch process stops automatically due to a very low etch rate for the oxidized porous silicon in the KOH solution. After removing the oxidized porous silicon layer the silicon layer beneath the oxidized porous silicon layer separates from the (110) silicon substrate and forms a flexural silicon strip.
The microchannels for holding the optical fibers and the cylindrical lenses are formed through deep reactive ion etching (DRIE).
As an alternative, the microchannels are also formed through anisotropic etching.
A method to form well-defined microchannels for holding optical fibers by etching of a (110) silicon substrate was described in U.S. Pat. No. 5,381,231. This U.S. patent was issued to the present inventor and has been incorporated here by reference of the present invention.
This method involves two fabrication steps. The first step is to form a plurality of cavity rows each cavity having side walls bounded by (111) crystal planes. An etching mask comprises a plurality of rhombic opening rows each opening having edges aligned with (111) crystal planes. Each row of rhombic openings comprises two different size rhombic openings that are alternatively disposed in a line. The second step is to remove very thin vertical walls formed between each two adjacent cavities by isotropic etching. After this fabrication step, each cavity row becomes a microchannel with two opposite zigzag side edges. This fabrication step also results in a plurality of ridge pairs disposed on the two opposite side walls of the microchannel. Each pair of ridges has a well-defined configuration. Particularly, the spacing between the two ridges can be precisely calculated.